1. Field of the Invention
The present invention relates to a liquid crystal display used in the industrial field, office automation or domestic use and the like and, specifically, to a liquid crystal panel frame used for the liquid crystal display. Further, the present invention relates to a liquid crystal panel assembly formed by encapsulating the liquid crystal into the same liquid crystal panel frame. Still further, the present invention relates to a method and apparatus for manufacturing the same liquid crystal panel assembly. More specifically, the present invention relates to a method of controlling the alignment of liquid crystal of the liquid crystal panel assembly in which ferroelectric or anti-ferroelectric liquid crystals are used.
2. Related Art
Liquid crystal displays (LCD), which can be made of lightweight and thin materials, are widely used as displays for small size electronic calculators, measuring instruments such as a tester or the like, and for displaying graphics or characters on them for decoration or POP purposes. Recently, it is also used as a large capacity thin terminal display adapted for a television set for displaying pictures dynamically in full colors by using thin film transistors (TFT), or for a personal computer or work station.
The foregoing various displays mainly utilize shuttering performance inherent to the liquid crystal, and the typical liquid crystals exhibiting the shuttering performance include twisted nematic type liquid crystals, super twisted nematic (STN) type liquid crystals or the like utilizing nematic phase. Further, there are also available ferroelectric liquid crystals or anti-ferroelectric liquid crystals or the like utilizing chiral smectic phase. These liquid crystal are described in detail in:
(1) "Liquid Crystal" written and edited by Kobayashi and Okano, Baifuhkan, 1985; PA1 (2) "Structure and Physical Properties of Ferroelectric Liquid Crystal" written by Fukuda and Takezoe, Corona Corporation, 1990; PA1 (3) "Next Generation Liquid Display and Liquid Crystal Material" edited by Fukuda; CMC corporation, 1992. PA1 (1) The method in which a pretilt angle is enlarged. That is, the axis of the liquid crystal molecule within the liquid crystal is previously tilted at a finite angle relative to the substrates, rather than being located in parallel to them. ("Next Generation Liquid Crystal Display and Liquid Crystal Material", edited under the supervision of Fukuda, page 85, CMC Corporation, 1992) PA1 (2) A suitable liquid crystal material is used and, further, the combination of the orientation film and the rubbing direction is made suitable (ibid, page 19), or PA1 (3) A ferroelectric liquid crystal material which is less in the folding degree of the liquid crystal layer (that is, the reduction of the spacing between the smectic phase layers) at the phase transition from the smectic A phase into the SmC* phase is used (ibid, page 37). PA1 (1) They occur at the portions where the SmA phase domains which have grown at different locations strike against each other (see sign X of FIG. 9). PA1 (2) They are the domains which grow fast in some angular direction relative to the rubbing direction at the portions where the orientating direction slightly differs from that of the perimeter surrounding the domain (see sign Y of FIG. 10). PA1 (3) They can often occur at the portions where the liquid crystal snakes and where the directions in which the smectic phase layers greatly differ (see sign Z of FIG. 11). PA1 (1) forming an orientation film on at least one of a pair of substrates opposed to each other; PA1 (2) applying a uniaxial alignment treatment to at least one of said orientation films; PA1 (3) forming a plurality of rectilinear spaces continuously in parallel between said substrates so as to extend in substantially parallel to the direction of said uniaxial alignment treatment, the rectilinear spaces each having an aperture at least one end portion, portions other than said aperture being sealed against liquid; and PA1 (4) encapsulating a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal into those rectilinear spaces; PA1 (5) retaining the encapsulated liquid crystal to a temperature presenting a high temperature phase; and PA1 (6) thereafter, sequentially cooling the encapsulated liquid crystal from one end portion of the rectilinear space to the other from a temperature presenting the high temperature phase down to a low temperature phase under the condition in which the temperature gradient is kept appearing along the direction of the uniaxial alignment treatment. PA1 (1) a stripe-shaped electrode in which a plurality of rectilinear electrodes is arrayed at predetermined intervals is formed on a pair of substrates, PA1 (2) an orientation film is formed on one or both of the stripe-shaped electrodes on each substrate, PA1 (3) uniaxial alignment treatment is applied to at least one of the orientation films, PA1 (4) rectilinear barrier members are formed between each rectilinear electrode of the stripe-shaped electrodes of the one of the substrates at the same intervals as those for the electrodes or at plural intervals so as to extend in substantially parallel to the direction of the uniaxial treatment, and PA1 (5) after those substrate are opposed so that the stripe-shaped electrodes lying on the pair of substrates intersect at a right angle with each other, the barrier member formed on one substrate is adhered on the other substrate to form a rectilinear Space sealed against liquid. PA1 (1) planar electrodes are formed on a pair of substrates, PA1 (2) an orientation film is formed on one or both of the planar electrodes on each substrate, PA1 (3) uniaxial alignment treatment is applied to at last one of the orientation film, PA1 (4) barrier members are formed at desired intervals on the planar electrode formed on the one substrate so as to extend in substantially parallel to the direction of the uniaxial alignment treatment, and PA1 (5) the barrier member formed on the one substrate is adhered to the other substrate to form a rectilinear space sealed against the liquid. PA1 (1) one in which both substrates are adhered by means of the stripe-shaped barrier member; PA1 (2) one in which both substrates are adhered by means of the dot-shaped member; and PA1 (3) one in which both substrates are not adhered by means of the stripe-shaped member.
The ferroelectric liquid crystal(FLC) was proposed by Clark et al (Japanese Patent Laid-Open 56-107216, U.S.P. specification No.4367924). Further, the anti-ferroelectric liquid crystal(AFLC) was found by A. D. L. Chandani et al., (Japanese Journal of Applied Physics, Vol 28, L1256 (1989). Since any of them has the so-called storage effect, it is expected that a large capacity display can be realized by driving a simple matrix addressing drive without using active elements such as thin film transistors(TFTs).
These liquid crystals exhibit complicated phase transitions such as, for example, the chiral nematic (N*) phase--the smectic A (SmA) phase--the chiral smectic C.alpha. phase--the chiral smectic C.beta. phase--the chiral smectic C.gamma. phase--the chiral smectic CA phase from the liquid phase, namely, the isotropic phase, which is the high temperature phase, as its temperature is lowered. Incidentally, depending on the kind of the liquid crystal, there are also phases which are not developed. For example, the anti-ferroelectric liquid crystal exhibits no chiral nematic phases. Further, the phase responsive to the electric field, which is necessary to the liquid crystal display, are chiral smectic phases which lie at the side of lower temperatures than the chiral nematic phase and are not so symmetrical with close to the crystalline state. To be more specific, with the ferroelectric liquid crystal, it is the chiral smectic C (SmC*) phase, with the anti-ferroelectric liquid crystal, it is either one of the chiral smectic CA (SmCA*) phase, the chiral smectic C.alpha. phase, the chiral smectic C.beta. phase, chiral smectic C.gamma. phase.
However, in order to put the display utilizing the ferroelectric liquid crystal or anti-ferroelectric liquid crystal into practical use, as also described in the foregoing literature (3), it is necessary to solve two problems simultaneously. One of them is that it is necessary to establish the method allowing a large area thin film comprising a defect-free chiral smectic phase to be mass produced, in particular, the technique for controlling the alignment of the liquid crystal. Further, the other is that it is necessary to accommodate the large area liquid crystal exhibiting a defect-free chiral smectic phase into a liquid crystal panel frame excellent in vibration- and shock-proof properties.
Conventionally, as the structure of the liquid crystal display, the method of utilizing the liquid crystal panel assembly as shown, for example, in FIG. 3 has been known. In this method, a pair of glass substrates 102, 103 provided with transparent electrodes 104, 105 is adhered to each other with a minor gap open to form a panel frame into which the liquid crystal is encapsulated. Then, a desired liquid crystal 101 is encapsulated into that minor gap to form a liquid crystal panel assembly. Further, polarizing plates 106, 106 are stuck to the liquid crystal panel assembly and, further, attachments such as driving print circuit boards or backlights and the like are packaged to form a liquid crystal display.
When the panel frame for encapsulating the liquid crystal is made, a multiplicity of spherical or cylindrical spacers 107 is disposed at one of the pair of glass substrates 102, 103, around which a seal portion 108 is printed in the form of a frame by means of screen printing or the like. Then, the other glass substrate is pressed against the foregoing glass substrate with an appropriate pressure with the spacers 107 and the seal portions 108 interleaved therebetween, and in this condition, the pair of glass substrates is entirely heated to heat and harden the seal portions 108 to adhere both of them.
Laminated on each of the opposed glass substrates 102, 103 are transparent electrodes 104, 105, an insulating film, a color filter and the like, as necessary. Further, on the uppermost portion contacting the liquid crystal, an organic film, for example, a polyimide film 109, 110 subjected to the uniaxial alignment treatment for orientating that liquid crystal, for example, a rubbing process, is formed. The width of the minor gap, that is, the cell gap is set to a desired value ranging from 1 to 10 .mu.m depending on the kind of the encapsulated liquid crystal. In particular, as regards the ferroelectric liquid crystal(FLC) or anti-ferroelectric liquid crystal(AFLC), the cell gap is set to 1 through 3 .mu.m, and more preferably, 1.5 through 2 .mu.m.
The encapsulation of the liquid crystal into the panel frame is carried out, for example, in the following manner. First, the panel frame is set within an evacuating apparatus and, after the interior of the panel frame is evacuated through an aperture portion, the aperture portion is blocked with the liquid crystal to be encapsulated. Thereafter, atmospheric air is introduced into the evacuating apertures to apply a differential pressure to the liquid crystal at the aperture portion to make that liquid crystal penetrate into the panel frame. The penetrating speed can be controlled by the pressure difference. The slowest speed is achieved when it is made to penetrate only with surface tension without applying the pressure difference. Incidentally, the interior of the liquid crystal panel frame shown in FIG. 3 constitutes a single continuous space without any partition, and if the liquid crystal penetrates through that internal space, then it can penetrate anywhere therein.
The penetrating temperature is one corresponding to the liquid phase of the liquid crystal to be encapsulated, and, with the ferroelectric liquid crystal or anti-ferroelectric liquid crystal, it will be on the order of 80.degree. C. to 120.degree. C. Thereafter, if the aperture portion is sealed and the liquid crystal is cooled again from the high temperature in a temperature controlled oven, then it follows phase transitions, such as the liquid phase--the chiral nematic phase--the smectic A phase--the chiral smectic C phase, to achieve a liquid crystal panel assembly having an aligned chiral smectic phase.
With this arrangement, since the upper and lower substrates 102, 103 are not adhered to each other at the positions other than the seal portions 108, if being locally pressed, gradual unevenness is occurred in substrates 102, 103, and thereby, the liquid crystal 101 within the liquid crystal panel assembly fluidizes. If the liquid crystal lies in the nematic phase, since the nematic phase is close to the liquid state, even if such a liquid crystal fluidization takes place, by releasing the pressure, the alignment of the liquid crystal is returned to the original order, so that no problem occurs. When the liquid crystal display incorporating the liquid crystal panel assembly is carried as a portable type or is used in the office on a daily basis, if a certain shock or physical stress is applied to the substrate, then the substrate is slightly deformed, but if they are released, it is returned to the original condition reversibly without causing any problem.
On the other hand, if the ferroelectric liquid crystal or anti-ferroelectric liquid crystal is encapsulated into the liquid crystal panel frame of this type of arrangement, and similarly, the substrate is deformed due to local pressure or impact, then the liquid crystal therein will fluidize. Since the ferroelectric liquid crystal or the like typically has a layered structure inherent to the smectic phase as shown in FIG. 7, once fluidization takes place to this, zigzag defects or turbulences will occur to the inherent layered structure, and these turbulences will never be eliminated. In this case, it will be necessary to heat the liquid crystal layers again to the isotropic phase and to cool down further for reorientation, but such an operation will be practically impossible. In order to prevent the turbulences of the liquid crystal layers, also in the process of packaging the attachments after its alignment is controlled, it is necessary to pay careful attention to their handling so that no shock or vibration is applied to the liquid crystal panel assembly, and further, some special devices such as shock absorbent material or panel surface protection member will become necessary also as the liquid crystal display. However, these will entail the reduction of productivity or increase of cost, thus narrowing the utilizing scope as the liquid crystal display.
In consequence, if the liquid crystal such as especially the ferroelectric liquid crystal is used, then it is necessary to use the panel frame excellent in vibration- and shock-proof properties which cannot cause excessive fluidization to the liquid crystal therein even if the substrate is pressed or suffers a shock. As a method which allows such a panel structure to be achieved, one in which the pair of substrates is firmly adhered to each other is publicly known. In the case where the displaying portion has an area more than 210 mm.times.295 mm (that is, A4 size), unless both substrates are adhered, any shape of spacer member can not practically employed because separation is occurred between both substrates when the alignment treatment being carried out, attachments being mounted on the panel assembly and the liquid crystal display being used in practice.
In the conventional type arrangement shown in FIG. 3, a technique in which adherent beads (that is, spherical pieces) are dispersed between both substrates to adhere them is disclosed in Japanese Patent Laid-Open 64-18126. Further, techniques in which a dot-shaped (that is, columnar) adherent member is formed on one of the substrates by photolithography to adhere both substrates more flexibly and stiffly are disclosed in Japanese Patent Laid-Opens 63-50817, 62-96925, 62-118323, 4-255826 and the like. Further, techniques in which a stripe-shaped adherent member is used are disclosed in Japanese Patent Laid-Opens 63-50817 and 63-135917 and the like.
The aim of adhering in the foregoing prior art is to retain the gap between the upper and lower substrates constant, or to dispose the spacers at positions corresponding to the non-pixel portion, and it is not allowed for therein that especially important matters with the present invention, that is, (1) how to penetrate the liquid crystal into the liquid crystal panel frame is specified, (2) the direction in which the volume of the liquid crystal contracts attended with cooling down is specified, (3) the direction in which the chiral smectic phase layer grows is controlled. As will be described later, for the substrates adhered in the dot-shaped manner, or using beads, it is impossible to achieve a defect-free chiral smectic phase. As regards ones using the stripe-shaped adherent member, there will be chances of getting the less defective chiral smectic phase, but it will be insufficient to merely adhere the substrates in the stripe-shaped manner.
Next, the initial alignment of the ferroelectric liquid crystal for a liquid crystal panel assembly in which both substrates are adhered by the adherent member and for one retaining the gap between both substrates simply by beads in place of using the adherent member is hereinafter described briefly. These liquid crystal panel assemblies have the cell gap (that is, the gap between the substrates) of about 2 .mu.m or less, and is subjected to a normal rubbing process. Further, these liquid crystal panel assemblies are cooled down in the oven or liquid after being penetrated with the ferroelectric liquid crystal.
Within layers of the SmC* phase obtained by cooling down the ferroelectric liquid crystal from the phase in the high temperature condition, some inherent abnormal alignment never fail to be found. These abnormal alignment include loop-shaped and line-shaped zigzag defects (as indicated by the numeral 113 of FIG. 4), tree-shaped defects generating at the portions where adjacent crystalline phase domains strike against each other (as indicated by the numeral 114 of FIG. 5), quasi-linear type defects (as indicated by the numeral 115 of FIG. 6) or the like. The crystalline phase domain means the smectic A phase which appears in the isotropic phase or the chiral nematic phase, and the chiral smectic C phase or anti-ferroelectric phase which appear in the smectic A phase.
When a non-adherent type spacer is dispersed between both substrates, or a dot-shaped spacer is randomly disposed to adhere both substrates to each other, loop-shaped zigzag defects are often found and, sometimes, the tree-shaped defects are found. Further, as shown in FIG. 27, when dot-shaped fine spacers 107 were regularly disposed to adhere both substrates, zigzag defects 116 running in synchronism with the cycle of the regularity of the spacers 107 were generated. Even if the non-adhere stripe-shaped spacers were used, a multiplicity of similar zigzag defects generated.
When both substrates are adhered by means of the stripe-shaped spacers, large zigzag defects which divide the panel surface into two or three are found and, further, a multiplicity of rectilinear or tree-shaped alignment defects are also found therein. In other words, it means that, if both substrates are adhered by means of the stripe-shaped spacers, although the amount of generated zigzag defects becomes small, such an adhesion is not enough to completely eliminate them. Further in this case, if the liquid crystal panel assembly is rapidly cooled down, narrow empty gaps which is caused by the liquid crystal layers contracting in the opposite directions are found between the adjacent stripe-shaped spacers.
If the alignment defect such as the zigzag defect or the like is present on the electrode one at all, then it is difficult to serve the liquid crystal panel assembly for practical purpose because when the refractive index relative to the rectilinear polarization differs even a little at both sides of zigzag defects, a slight shading is generated, or when the liquid crystal display is driven, the defects themselves constantly flicker and further new defects are likely to occur. This is also the case even when the empty gap takes place.
The structure of the zigzag defects and several methods to eliminate them are described in the foregoing document (3), in which the zigzag defects are considered to occur necessarily because the SmC* phase exhibits the chevron structure shown in FIG. 7. This chevron structure refers to a phenomenon in which the liquid crystal layer S of chiral smectic phase is bent in the shape of "&lt;&lt;". This bending direction is not uniquely determined, but there are two types, one 111 in which the liquid crystal layer directs in the leftward direction as viewed in Figure, and the other 112 in which the liquid crystal directs in the rightward direction, and the zigzag defect 113 occurs between each boundary. As illustrated in FIG. 8, if the liquid crystal layer S of chiral smectic phase takes an ideal bookshelf structure, then it is considered that the domain-shaped zigzag defects do not happen. However, even in this case, there is a chance that the tree-shaped or linear defects are developed. Incidentally, referring to FIGS. 7 and 8, the sign "K" denotes the boundary between the substrates and the liquid crystal layer.
In order to eliminate the zigzag defects, a method of fixing the bending direction of the liquid crystal layer of the chevron structure has conventionally been known as follows.
However, in the method of (1), the oblique vapor deposition process is adopted, and it takes practically no effect for the alignment control with a large area liquid crystal display having an area more than 181 mm.times.256 mm (that is, B5 size). Further, the processes of (2) and (3) are effective only for specific materials, and cannot be universally applied to every material. In further addition, if the bending directions of the liquid crystal layers should be successfully controlled in a one direction according to each of the foregoing processes, it is not clear for us to also eliminate the defects caused by the collision of the liquid crystal phase domains which generate at different locations in the cooling process of the liquid crystal, or defects caused by the contraction of the volume of the liquid crystal.
As an alternative to the foregoing, another method is proposed in Japanese Patent Laid-Open 2-18 by the applicant in which, after the layers of the SmC* phase is formed, the generated zigzag defects are locally heated and, further, with the same heating area shifted, are expelled from the effective display area. However, this attempt is to remove the zigzag defects after the smectic layers are formed, and does not achieve smectic layers which are defect free from the very beginning, as in the present invention.
As a further alternative, an example in which the liquid crystal panel assembly encapsulated with liquid crystal is cooled down along the rubbing direction is disclosed in Japanese Patent Laid-Open 61-182017. This is intended to improve the alignment performance of the nematic phase of a hybrid type nematic liquid crystal which is subjected to a parallel alignment process on one side and to a vertical alignment process on the other side, and differs from the crystal growth of the chiral smectic phase, to which the present invention is directed.
In the techniques disclosed in the foregoing Japanese Patent Laid-Opens 2-18 and 61-182017, the interior of the panel frame to be encapsulated with the liquid crystal constitutes a single space free of any partition, and hence, the desired defect free chiral smectic phase cannot be achieved by any means.
As a still further alternative, there have been known a method in which less defective layers of the chiral smectic phase are induced, by shifting the temperature gradient, onto the electrode with the crystalline cross-section of PET (polyethylene terephthalate) film as the starting point (see, for example, "Structure and Physical Properties of Ferroelectric Liquid Crystal" written by Fukuda and Takemori; Corona Corporation, 1990 page 234). However, this process does not have in mind at all that the uniaxial alignment process, for example, the rubbing process is carried out by using the orientation film, so that it cannot be used for reference if one tries to obtain the defect-free chiral smectic phase by using the rubbing process. In practice, such a conventional method cannot be applied at all to the large area liquid crystal panel assembly, to which the present invention is directed.
On the other hand, the anti-ferroelectric liquid crystal differs from the ferroelectric liquid crystal in that the smectic A (SmA) phase directly precipitates from the isotropic phase, that is, nucleation happens because the anti-ferroelectric liquid crystal does not have the chiral nematic phase. The alignment defects can be visually observed when the phase changes from the isotropic phase to the smectic A (SmA) phase, and when it undergoes from the SmA phase to the anti-ferroelectric phase, for example, the SmCA* phase. The defects observed as the phase changes from the SmA phase to the SmCA* phase are similar to those for the ferroelectric liquid crystal.
When the smectic phase A (SmA) phase precipitates from the isotropic phase, although it precipitates first growing in parallel to the rubbing direction, generally, it precipitates spreading at the same or slightly slower speed also in the direction perpendicular to the rubbing direction. The alignment defects observed at this time are as follows.
The defects as in (1) take the form of laces extending in the substantially perpendicular direction relative to the rubbing direction, the laces being small but large in number. The reason why the defects of (2) develop is unknown, but they are large inters of area.
Although the defects of (1) and (3) are similar, their magnitudes of deviation differ. With reference to the defects of (1), the directions in which the layers of both domains run with respect to the defects are basically the same because they are define in the rubbing direction. Further, these defects, since formed due to the growth of both domains from different directions, deviate slightly. These defects remain as such also in the low temperature phase of the liquid crystal, for example, the anti-ferroelectric phase.
With reference to the defects of (1) and the alignment defects of the ferroelectric liquid crystal, it is shown in, for example, Japanese Patent Laid-Open 63-303323 that, if those defects are small in number, they can be removed by supplying rectangular electrical waves of about 100 through 300 Hz for several hours. However, such a technique utilizing the electric field is not also practical.
As described above, conventionally, a technique which can make to grow mono-domain layers constituted with defect free chiral smectic phase based on a certain sort of law has not been known. That is, when the liquid crystal panel assembly is made by utilizing the conventional alignment controlling method, at least one of the zigzag defects, tree-like alignment defects and linear alignment defects will necessarily be generated over large layers of the chiral smectic phase.